Implemented dynamic main menu backgrounds.
- This can be switched off in the configuration file.
- Credits: https://osu.ppy.sh/forum/t/98954

Other changes:
- Removed OsuFile setters/getters from Game state, replaced with a 'getOsuFile()' method in MusicController (a more logical location).

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>